Nonalcoholic fatty liver disease (NAFLD) is marked by accumulation of fat in liver cells with accompanying inflammation and variable degrees of cell injury and fibrosis. When cell injury and fibrosis are present, the disease has a potential to progress and is referred to as nonalcoholic steatohepatitis (NASH). The etiology of NASH is not clear, but most patients are overweight or obese and have either insulin resistance or frank diabetes. Because of this association with obesity and diabetes, therapies for NASH have focused upon weight loss and improvement in insulin resistance. Starting in 2002, we conducted a series of clinical research studies in NASH. An initial study focused on the thiazolidinediones (TZDs), insulin sensitizing agents that are used widely in diabetes. In a pilot study, 22 non-diabetic patients with NASH were treated with pioglitazone (30 mg daily) and underwent extensive testing for metabolic status, body composition and liver disease (including liver biopsy) before and at the end of 48 weeks of therapy. Improvement was assessed by strict histological criteria. A preliminary report on this study showed that two-thirds of patients improved on therapy as judged by strict histological criteria. Improvements were accompanied by a marked decrease in hepatic fat despite an overall increase in body weight and total body fat. Thus, the effects of TZDs in NASH appeared to be due to the redirection of fat storage from the liver and central sites to the periphery. The histological improvements in the liver were not just in amount of fat (steatosis), but more strikingly in cell injury, inflammation and fibrosis. In follow up of this study, samples from patients were tested for a battery of cytokines and adipokines. Histological improvements correlated most clearly with changes in adiponectin, a adipokine that improves insulin signaling and induces maturation of adipocytes. When pioglitazone was stopped, the serum biochemical and histological features of NASH were reversed, histological scores returning to baseline by a year after discontinuation of pioglitazone. Importantly, the weight gain that occurred during pioglitazone therapy was not reversed;so that patients who received a one-year course of pioglitazone no longer had the histological benefit but were considerably heavier than before they were treated. These finds indicate that long-term improvement in NASH would require long-term therapy with a TZD and that the weight gain that often accompanies TZD therapy is likely to ultimately reverse any benefit. Recently, we completed a prospective, open-labelled study of metformin as therapy for NASH. A total of 28 patients with NASH were enrolled. The design of the study was similar to that for pioglitazone, in that patients underwent extensive evaluation of body composition, metabolic status, insulin sensitivity and liver disease (including liver biopsy) before and at the end of a 48 week course of metformin (2000 mg daily). The primary endpoint was histologic improvement, defined as a 3-point improvement in the NASH activity index. Of 28 patients enrolled, 26 (13 females;average age 44 years) completed 48 weeks of treatment and underwent repeat metabolic studies, imaging and liver biopsy. Thirty percent achieved a histologic response. Most patients lost weight, the average being 6 kg. There was a marked association between weight loss and improvements in NASH activity index and ALT levels (both, p <0.01). Insulin sensitivity also improved, but the degree of change did not correlate with histologic improvement. Thus, metformin leads to improvements in liver histology and ALT levels in 30% of patients with NASH probably by its effects in causing weight loss. Future studies in NASH will be directed at weight loss as a means of improving this liver disease. Possible therapies that are being considered include use of cannabinoid receptor antagonists (such as rimonabant) and use of high doses of metformin. To define the genetic linkage between NAFLD, obesity, and metabolic syndrome, we have initiated several genetic studies of NAFLD. Obesity is an important correlate of serum alanine (ALT) and aspartate (AST) aminotransferase levels. In the first study, we sought to examine the relations between parental obesity and the serum ALT and AST levels among offspring in a community-based sample. Participants (n=1732) of the Framingham Offspring Study (50% women, mean age 42 years) who had serum ALT and AST measurements and both parents in the Framingham Original cohort, were studied. Study participants were grouped into early-onset parental obesity n=193 (at least one parent obese), later-onset parental obesity n=460, and no parental obesity n=1079 subgroups. The association between elevated ALT or AST and parental obesity was tested using generalized estimating equations to account for familial correlations. In multivariable analysis including adjustment for offspring obesity, significantly higher ALT was observed among individuals with paternal early-onset obesity as compared to those without paternal obesity (p-value=0.02). Offspring with early-onset paternal obesity were more likely to have elevated ALT levels compared with those without paternal obesity (odds ratio OR 1.75 (95% CI 1.06-2.89;p=0.03). There was no association with elevated ALT among offspring with maternal early-onset obesity (OR 1.10, 95% CI 0.76-1.59;p=0.61). There was no association between parental obesity and serum AST levels. Early-onset paternal obesity, but not maternal obesity, increases the odds of elevated serum ALT levels in the offspring, suggesting a predisposition to developing elevated serum ALT levels that may be mediated through familial early-onset obesity. Currently we are also collaborating with a large NASH consortium to identify potential genetic markers for NAFLD.